为什么 package-info.java 有用？
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Why is package-info.java useful?
提问by Socrates
When I run CheckStyle over my Java project it says Missing package-info.java file.for some classes, but not all of them. I can't really figure out why this message appears only sometimes. Furthermore my project runs perfectly fine without the package-info.java.
当我在我的 Java 项目上运行 CheckStyle 时，它会Missing package-info.java file.显示某些类，但不是所有类。我真的不明白为什么这条消息只是有时出现。此外，我的项目在没有 package-info.java 的情况下运行得非常好。
What does the package-info.java do? Do I really need it for my Java projects?
package-info.java 有什么作用？我的 Java 项目真的需要它吗？
采纳答案by m-szalik
It is used to generate javadocs for a package.
它用于为包生成 javadoc。
/**
* Domain classes used to produce .....
* <p>
* These classes contain the ......
* </p>
*
* @since 1.0
* @author somebody
* @version 1.0
*/
package com.domain;
Will generate package info for com.domainpackage:
将为包生成包信息com.domain：
Example result: https://docs.oracle.com/javase/7/docs/api/java/awt/package-summary.html
结果示例：https: //docs.oracle.com/javase/7/docs/api/java/awt/package-summary.html
回答by JB Nizet
A package-info.java file allows adding javadoc to document a whole package. See http://docs.oracle.com/javase/7/docs/api/java/applet/package-summary.htmlfor example.
package-info.java 文件允许添加 javadoc 来记录整个包。例如，请参见http://docs.oracle.com/javase/7/docs/api/java/applet/package-summary.html。
If you don't care about missing package documentation, then ignore the warning or disable the JavadocPackage check.
如果您不关心缺少包文档，则忽略警告或禁用JavadocPackage check。
回答by mdhirsch
Annotations
注释
Another good reason to use package-info.java is to add default annotationsfor use by FindBugs. For instance, if you put this in your package-info file:
使用 package-info.java 的另一个很好的理由是添加默认注释以供FindBugs使用。例如，如果你把它放在你的 package-info 文件中：
@DefaultAnnotation(NonNull.class)
package com.my.package;
then when findbugs runs on the code in that package, all methods and fields are assumed to be non-null unless you annotate them with @CheckForNull. This is much nicer and more foolproof than requiring developers to add @NonNullannotations to each method and field.
然后当 findbugs 在该包中的代码上运行时，所有方法和字段都被假定为非空，除非您使用@CheckForNull. 这比要求开发人员@NonNull为每个方法和字段添加注释要好得多，也更万无一失。
回答by Rene Mazala
Not only some findbugs annotations, but a lot of java annotations in common libraries have the java.lang.annotation.ElementType.PACKAGEtype as one of the possible values of their own java.lang.annotation.Targetannotation, e.g.:
不仅是一些 findbugs 注释，而且很多公共库中的 Java 注释都将java.lang.annotation.ElementType.PACKAGEtype 作为它们自己的java.lang.annotation.Target注释的可能值之一，例如：
com.google.gwt.core.client.js.JsNamespace
com.querydsl.core.annotations.Config
com.sun.xml.bind.XmlAccessorFactory
groovy.transform.BaseScript
java.lang.Deprecated
javax.annotation.Generated
javax.xml.bind.annotation.XmlAccessorOrder
org.hibernate.annotations.TypeDef
net.sf.ehcache.pool.sizeof.annotations.IgnoreSizeOf
org.apache.hive.common.HiveVersionAnnotation
org.apache.wicket.authroles.authorization.strategies.role.annotations.AuthorizeAction
org.codehaus.commons.nullanalysis.NotNullByDefault
org.eclipse.persistence.oxm.annotations.XmlNameTransformer
org.glassfish.jersey.Beta
org.jgroups.annotations.Experimental
and much more.
以及更多。
This package-info.javafile would be the file, where you can place such annotations (along with the javadoc).
该package-info.java文件将是您可以在其中放置此类注释（以及 javadoc）的文件。
回答by subodhkarwa
The package-info.javais a Java file that can be added to any Java source package. It is used to provide info at a "package" level as per its name. It contains documentation and annotations used in the package.
该package-info.java是可以添加到任何Java源代码包的Java文件。它用于根据其名称在“包”级别提供信息。它包含包中使用的文档和注释。
javadoc example is already provided in the answer, the below part explains how it works incase of annotations.
答案中已经提供了 javadoc 示例，以下部分解释了在注释的情况下它是如何工作的。
For example, in the below file it is used to "substitute" the occurance of joda.time.DateTime with org.jadira.usertype.dateandtime.joda.PersistentDateTime
例如，在下面的文件中，它用于“替换” joda.time.DateTime 与 org.jadira.usertype.dateandtime.joda.PersistentDateTime 的出现
@TypeDefs({
@TypeDef(name = "PersistentDateTime", typeClass = PersistentDateTime.class, defaultForType=DateTime.class)})
package xyz.abc;
import org.hibernate.annotations.TypeDef;
import org.hibernate.annotations.TypeDefs;
import org.jadira.usertype.dateandtime.joda.PersistentDateTime;
import org.joda.time.DateTime;
There are a number of annotations available with which can be used to perform different things at "package" level. It can be found at https://docs.jboss.org/hibernate/orm/3.5/api/org/hibernate/annotations/package-summary.html
有许多注释可用于在“包”级别执行不同的操作。它可以在https://docs.jboss.org/hibernate/orm/3.5/api/org/hibernate/annotations/package-summary.html找到
